fix(web): follow a blank session's preset switch in the slash catalog

Presets own the rows that decide what a session's `/` menu contains, but
both browser catalogs cache per session and had no invalidation edge for a
recompose: `commands/changed` is registry-wide and recomposing registers
nothing, so the menu kept serving the composition the session no longer ran.

The host stream now frames the logged `agent-preset/selected` commit as
`host/session-preset-changed`; the runtime bridges it to the typed
`session/preset-changed` event, `ui-command` soft-refreshes that session's
directory key and `ui-skill` invalidates its catalog entry.

Reaching the host on a second switch was a separate defect: the list-row
identity guard compared every summary field except `agentPreset`, and the
merge keeps the row's `updatedAt`, so a switched row looked unchanged and
served its cached instance forever. The hero chip compares the pick against
that row, so switching back to the creation-time preset sent no RPC at all.

packages/client/ui-skill/tests/browser-plugin.spec.ts

@@ -263,6 +263,21 @@ describe('catalog cache', () => {
     expect(payloads).toHaveLength(2)
   })
 
+  it('session/preset-changed clears only the recomposed session', async () => {
+    const { list, payloads } = countingList()
+    const { ctx, source } = await bench(list)
+    await source.candidates(proj('s1'), req(''))
+    await source.candidates(proj('s2'), req(''))
+    expect(payloads).toHaveLength(2)
+    // The catalog a preset supplies is the preset's; the other session's
+    // composition did not change, so its cached catalog still holds.
+    ctx.emit('session/preset-changed', sid('s1'), 'minimal')
+    await source.candidates(proj('s1'), req(''))
+    await source.candidates(proj('s2'), req(''))
+    expect(payloads).toHaveLength(3)
+    expect(payloads[2]).toEqual({ sessionId: 's1' })
+  })
+
   it('connection/reset clears every cached session', async () => {
     const { list, payloads } = countingList()
     const { ctx, source } = await bench(list)